Regarding altenatives at that scale, I've not been involved in a project at that kind of scale, biggest SAP implementation I was involved in was ~10k employees and Alphabet is sitting at ~130k right now. I don't want to dive too much into speculation, perhaps Google will get a good outcome with this approach and since they're coming from a bad place with Oracle the future might still be crap but perhaps just a bit less so.
The biggest issue with procurement of SAP and it's ilk is it shifts a fundamental burden from procurement to implementation. With a defined software/SaaS solution that's ready to go out of the box, you get to answer a lot of the yes/no functional questions at procurement stage. It can do it or it can't do it and you get to decide how important that is to you. With SAP, every requirement is answered with 'it can do that' with the massive asterix that to make it do what you need it's all going to need to be customized to do so. That opens you up to a lot of risk IMO.
It's cheaper, it's faster, it doesn't need training and cheat cards, it's self explanatory, and you can maintain it by yourself. Plus you know it inside out. Plus you can add much better features, like graphical reports on top of it easier.
So therein lies the issue - Evaluating a fully customizable software product that will be critical to your business is very difficult to do before it is actually crafted into what you have in mind. So the procurement and implementation processes are often an uphill battle against magical thinking where people are imagining a certain wonderful outcome but without careful, critical attention what you'll end up with is a heap of shit.
I have a policy of not working for organizations that have either oracle or SAP in the core of their environment, unless my purpose in coming onboard is to move them off it. This limits me in many ways, but I know from experience I'm avoiding a significant amount of pure bullshit, enough to make it worth the rule.
*if you're willing to fund an as-yet-unknowable level of customization effort to reach your organization's particular nirvana.
tl;dr the future SAP has sold to Google may indeed look better than the Oracle reality they're currently living, but it doesn't necessarily follow that their SAP reality will be any good. Such are the perils of procurement.